Not really sure but maybe this can help: http://northstar-www.dart...nd_Eigenvectors.html Look for "compute the eigenvalues and eigenvectors of a real, nonsymmetric n-by-n array"

Thanks for the link, although it just shows examples for the 2D case (i.e. n-by-n), and uses the algorithms provided in the standard IDL distribution. I need an extension to this to allow me to compute the eigenvalues for an n-by-n-by-n array... I have seen some Matlab and C++ code for this, but really don't fancy having to translate it, particularly if there's already something tried and tested in IDL already out there.